Concentration of aqueous solutions is a common process in many industries. The technique widely used for the concentration of aqueous solutions is evaporation. However, an efficient evaporation process must be performed under boiling conditions at higher temperatures, which may result in loss and/or damage of certain volatile or heat-sensitive materials in the solutions. Instead of turning water into steam, fractional freezing is a process by crystallizing water in ice at temperatures below freezing point of the solutions, hence, making aqueous solutions concentrated. Purpose of fractional freezing has been to obtain concentrated solutions, which may include colloidal solution, true solution and or mixture of solution, purified water etc. Fractional freezing has many advantages over evaporation because of the lower process temperature. At a lower temperature flavors, aromas, nutrients and other valuable components in the original materials can be kept without loss. Moreover, at a lower temperature, destruction of heat-sensitive substances can be avoided. Therefore, with fractional freezing extremely high quality products can be obtained. The fractional freezing process can be applied for concentrating food, beverage, dairy, biochemical, nutriceutical, pharmaceutical, chemical, and environmental industries.
Theoretically, the latent heat of sublimation from water to ice is about 80 kcal/kg and is only one seventh of that from water to steam about 540 kcal/kg. So the process of fractional freezing there is a great potential to save energy for concentrating aqueous solutions. Freeze concentration contains steps of refrigeration of solution, crystallization of ice and separation of ice crystals from the mother liquid. To make a freeze concentration technique commercially feasible, aqueous solutions must be efficiently and economically refrigerated and large, uniform ice crystals, which are easily separated from the solution, must be efficiently obtained. However, ice crystallization is a complex phase transition and control of ice crystallization is very difficult due to the complexity. Therefore, the major difficulties for freeze concentration are in two interrelated aspects. Firstly, it is hard to separate ice crystals from the concentrated solution because of small size of ice crystals obtained. Secondly, process of obtaining large ice crystals proceeds slowly and the efficiency is low.
U.S. Pat. No. 4,666,456 relates to a process for fractional freezing, which includes continuous partial crystallization of a compound from a liquid mixture in which the mixture fed through a cascade of a plurality of cooling sections. These cooling sections connected in series, and the temperature of each subsequent one of the cascading cooling sections is lower than that of the preceding one. U.S. Pat. No. 4,885,016 described method and apparatus for superpurifying crystallizable substances by a multistage recrystallization procedure, which includes measures to control reflux ratio conditions by providing metering procedures that control quantities of crystals and mother liquor reflux materials that are transferred according to the method, and apparatus. In each stage, a crystallizable substance is frozen and recrystallized, and the crystals separated from the mother liquor. U.S. Pat. No. 5,127,921 discloses method and apparatus for superpurifying crystallizable substances by a multi-stage recrystallization procedure which includes measures to control reflux ratio conditions by providing metering procedures that control quantities of crystals and mother liquor reflux materials that are transferred according to the method and apparatus. U.S. Pat. No. 4,112,702 discloses freeze desalination and concentration through heat exchanger and refrigeration. U.S. Pat. No. 4,332,140 tells about an apparatus for the continuous operation of concentrating aqueous solution through counter current crystallizer.
To the best of the Applicant's knowledge there is no process available for concentrating the dilute solution in a single fractionation column.